When me mam was a nurse MSRIP she had to bring folk back that were trying to escape the Doctors. She said they had 3 minutes from flatlining before the brain turned Cabbage like. She had her stories but it makes you wonder what happens in that time before they catch you againe eh dunt it.
I know you know this, but ill say it all the same! : )
Just because your heart is not beating, does not mean you are dead. All the heart is is a pump, to supply the rest of the mechanism with oxygen (etc) and take away the wastes.
If you turn off your reactors pump, that does not put the reaction into immediate stasis. The instant after the pump is off, not a lot will change, and then things will slow in the following instants.
Of course, if you believe the fairy tales about 'souls' and the like, it all gets more difficult to comprehend. When does the soul leave? If someone gets significant brain damage, where's the soul then?
But, if you dont need a deity (etc) to justify your existence, and offer you an alternative to accepting your absolute and total insignificance, 'you' dont go anywhere - the order in your brain just starts to fall apart, along with all that you think you are.
Catch it quick enough, and youre back. Leave it too long, and youre soup.
Im told i was in hospital for a couple of weeks once. I was out for a while (im told) before my first memories of the occasion. Classic sci-fi stuff - having to ask people what year it was (etc). A load of people have asked "where do you think you were in that time" or similar.
I wasnt anywhere. The machine inside my skull was like a piece of damp paper that couldnt be written on, nothing more. I have no delusions of grandeur.
Anything more, and youre back to fairies living down the bottom of the garden ; )